1. Field of the Invention
The invention relates to steam power plant turbine blade vibration monitoring and control systems and methods. More particularly, the present invention is a blade vibration monitor backpressure limiting system (BVMBLS) that in addition to direct blade vibration and condenser backpressure monitoring utilizes other plural types of other parallel, real time monitored power plant operation state (OS) information that influence blade vibration. The BVMBLS references a base of stored knowledge that associates OS information with turbine blade vibration, and determines in real time a likelihood of whether any of the monitored OS, alone or in combination with other types of monitored OS, is indicative of a turbine blade vibration safe operation (SO) state. The BVMBLS determination is utilized to increase or reduce incrementally the monitored turbine's power generation load, so that power output and efficiency is enhanced while turbine blade vibration is in a safe operation (SO) state.
2. Description of the Prior Art
The last row of rotating blading in low pressure steam turbines contains the longest rotating blades in the turbine train. For this reason they are the most susceptible to aero elastic induced vibration. These last row blades are also subject to a vacuum back pressure on the downstream side, due to the condenser altering pressure characteristics as steam changes phase back to liquid. Generally, condenser backpressure increases with the condenser liquid phase temperature, and that temperature increases with power plant operational output load. For these reasons the backpressure critically influences the vibration characteristics of last row LP blading. If the backpressure is allowed to get too high the rotating blading can enter into a high vibration condition due to a variety of aero elastic events such as buffeting, stall flutter, and unstall flutter. The exact point of excessive blade vibration initiation cannot be predicted with certainty and can change rapidly, but its general probability can be forecasted within a band confidence within known backpressure/load operation states. The confidence of a safe operation state is determined by a combination of engineering calculations, monitored OS data, and historical data for the monitored turbine and other turbines.
Excessive blade vibration and its resulting high cycle fatigue of the LP blading can result in cracked blades. Routine inspections are typical and expensive repairs are required if cracked blading is discovered. If a crack propagates until the blade is liberated during operation, sometimes referred to as “throwing a blade”, there is a significant chance that the thrown blade will cause damage to the LP turbine, possibly resulting in a rotor vibration induced failure of the whole turbine train, and damage to other plant equipment. Traditionally excessive operational turbine blade vibration has been avoided by conservative establishment of condenser backpressure thresholds. As backpressure approached a threshold limit power plant load was reduced, which also reduced power production capability and operating efficiency. If the backpressure exceeded the threshold the power plant control system would shut or reduce steam volume and/or pressure to the turbine, resulting in a “turbine trip” that essentially takes the turbine off line for further power generation until the backpressure event was remedied.
Traditional threshold limits on backpressure are calculated based on manufacturer and power plant operator operational experience and the design of the turbine, and are intended to minimize the chance of LP blade vibration and the resulting damage. These calculations are very conservative because the exact point of initiation of vibration is impossible to determine and the negative consequences of running a turbine excessive high vibration are great. At any given moment during power plant operation the operation safety (OS) margin based on any particular set of power plant operating conditions is not presently known. In addition to condenser gross backpressure operational monitoring steam turbines have been outfitted with a Blade Vibration Monitors (BVMs) for real time measurement of vibration. When the BVM determines that turbine blade vibration exceeded a defined threshold it caused the plant control system to trip the steam turbine. Typically BVM systems have been utilized in reaction to a prior blade cracking incident or for operational qualification of new turbine designs. The BVMs have also been used to verify that there is not excessive blade vibration during operation within recommended condenser backpressure limits.
Known blade vibration operational controls relying on rigid backpressure threshold rules, with or without supplementary BVM, are essentially “binary” in operation or alarming: for example, the turbine operational control system only alarms when the backpressure or BVM measured blade vibration exceeds first or default threshold settings, but trips the steam turbine when a second set of higher threshold settings is exceeded. The BVM and backpressure monitoring systems are essentially passive until the predefined thresholds are exceeded, and do nothing to optimize turbine power generation load. A rigid, threshold rule base detection system relying on a single digital adequate/excessive condenser backpressure or blade vibration input does not optimize power plant load and efficiency. Rather, backpressure detection alone, or with an actual BVM system only avoids operation state vibration triggering events that are sufficiently grave to warrant load reduction alarming or automatic steam turbine tripping.
Thus, a need exists in the art for a steam power plant turbine blade vibration monitoring and backpressure limiting system that can in real time monitor plural types of power plant operating state (OS) inputs, in addition to blade vibration and condenser backpressure, to evaluate whether the inputs separately or in combination are indicative of a safe operation (SO) condition by referencing stored information resources. If a SO state is determined, to then incrementally increase the plant power generation load to increase plant operation efficiency. This increase is subject to other limiting conditions to load, steam quality or other parameters in the power plant. Conversely, if a safe SO condition is not indicated, incrementally decrease the load to avoid an increased potential for turbine blade damage. In this manner power generation would be optimized through dynamic monitoring and control rather than relying on rigid binary blade vibration decision making that causes the power plant to operate at less than maximum or optimal potential power generation load and efficiency. Dynamic control also would reduce risk of power disruption attributable to turbine tripping.